1. Field of the Invention
Methods consistent with the present invention relate to a channel time allocation method in a distributed wireless personal area network which is based on a mobile ad-hoc network, for data slot allocation of media access control (MAC).
2. Description of the Related Art
A wireless personal area network (WPAN) operates in a personal area within about I Om. The Institute of Electrical and Electronics Engineers (IEEE) is working on the standardization of the WPAN. Ultra wide band (UWB) technology can provide a data rate more than hundreds of Mbps in the WPAN. In the WPAN, all devices share a communication medium. This requires a MAC to control the media access of the devices. The MAC in a broad sense includes how to access the network, how to transfer data to another device at a desired data rate, and how to optimally use the media.
The MAC for the WPAN can be designed in either the centralized approach or the distributed approach. In the centralized approach, each device operates for the entire network so as to manage and control the media access of all of the devices. The devices require a centralized coordinator for the sake of the media access such as channel time allocation. In the distributed approach, the media access is uniformly distributed to all of the devices in the network. The devices share the burden of managing their media access.
FIG. 1 depicts a conventional WPAN. In FIG. 1, the network supports the centralized MAC approach based on IEEE 802.15.3, and includes a piconet. One device in the piconet is a piconet coordinator (PNC) 10. The PNC 10 provides functions to admit the network access of a device, to allocate a channel (time slot) to transfer data to another device, and to synchronize the devices. This is the centralized ad-hoc WPAN.
FIG. 2 depicts a WPAN without a centralized coordinator. Referring to FIG. 2, the WPAN includes a plurality of devices indicated as dots. Circles around the devices represent a communication range of the relevant devices.
The network in FIG. 2 supports the distributed MAC approach. All of the devices collaborate and share required information for the MAC such as the approval of the joining of a new device, the channel time allocation to each device to transfer data to another device, the synchronization, and the power reduction. Accordingly, none of the devices in the network is the dedicated coordinator.
The distributed MAC approach depends on a timing called a superframe. The superframe has a fixed length of time and is split into a plurality of time windows called time slots. The time slot is also called a medium access slot (MAS). Most of the time slots are used to send a beacon by the devices. The rest of the time slots are used to transfer data. The slots for the beacon transmission are beacon slots, and the slots for the data transfer are data slots. The length of a beacon period (BP) may be less than that of a data period. The beacon slots are distributed over the slots of the superframe or put in front of the superframe. The number of beacons may be fixed, or variable when it is implemented according to other distributed MAC approaches.
FIG. 3 depicts a conventional superframe format. The superframe format, as shown in FIG. 3, is based on the multiband orthogonal frequency division modulation (OFDM) Alliance draft v0.5. The superframe consists of 256 MAS's. Reference numeral a10 indicates the beacon period comprising the beacon slots, and reference numeral a20 indicates the data period comprising MAS's usable by other devices to transfer a stream (data) to the other devices in the network. The length of the superframe is 64 ms and the length of each MAS is 256 μs.
Information relating to the superframe is broadcast in the beacon slots forming the beacon period assigned to each device. Neighbor devices utilize the broadcast information in the next superframe. The start point of the superframe is determined by the start of the beacon period, and is defined to a beacon period start time (BPST).
The devices need to search for free beacon slots that are unused in the beacon period so as to send their beacons. Furthermore, the free data slots are required for mutual communication of the devices. However, in the situation that free data slots are absent, the devices cannot transmit and receive the stream when needed. As a result, new methods are required to seamlessly transmit and receive the stream even when a free data slot is not available.